The present invention relates to a solid-state laser device with a solid-state laser medium arranged between a total reflection mirror and an output mirror.
Solid-state lasers such as YAG lasers have been used widely in the field of laser processing because of their small size and ease of use. Recently, solid-state lasers have become widely used in the measuring and medical fields. In use, solid-state lasers must produce high-quality beams which can be focused on a small area. The quality of a laser beam is generally measured by the product ".theta.d" where .theta. is the beam divergence angle and "d" is the beam diameter at the beam waist. The quantity .theta.d is conserved even if a laser beam is converted by a lens. In other words, .theta.d=.theta..sub.1 d.sub.1 holds, where .theta..sub.1 is a converging angle when a laser beam is focused by a convex lens, and d.sub.1 is the beam-waist diameter at the focal point. Thus, if .theta..sub.1 is constant, d.sub.1 becomes smaller when .theta.d is reduced, so that a laser beam can be focused onto a smaller area. If the laser beam is focused more tightly, a high energy density is obtained, thereby improving the processing performance. Also, for transmission of a laser beam through an optical fiber, a fiber with smaller diameter can be used. A light beam output from such a fiber also has a small cross section.
The following are among techniques for reducing the .theta.d value:
(a) increasing the resonator length, PA1 (b) including a beam expander in a resonator, PA1 (c) using an unstable resonator, PA1 (d) using a slab-shaped laser medium in a solid-state laser, and PA1 (e) outputting a laser beam from a pinhole in an output mirror.
With reference to FIG. 2, the stability of an optical resonator including a total-reflection mirror 22 with a radius of curvature R.sub.1 and an output mirror 23 with a radius of curvature R.sub.2 sandwiching a lens 21 with a focal length "f" at respective distances a.sub.1 and a.sub.2 may be expressed by the resonator parameters EQU g.sub.1 =1-a.sub.2 /f-a.sub.0 /R.sub.1 and EQU g.sub.2 =1-a.sub.1 /f-a.sub.0 /R.sub.2, where EQU a.sub.0 =a.sub.1 +a.sub.2 -a.sub.1 a.sub.2 /f.
Using g.sub.1 and g.sub.2, a condition that laser light is confined in the resonator is expressed by formula (1): EQU 0&lt;g.sub.1 g.sub.2 &lt;1 (1)
If g.sub.1 and g.sub.2 change, .theta. and d also change.
In a device using a conventional rod-shaped laser medium, the temperature is highest at the rod center, and decreases toward the periphery. Thus, because the laser medium acts as a convex lens (thermal lens), the device has a resonator as represented by FIG. 2. Since the focal length "f" of the thermal lens changes with the input energy, the resonator parameters g.sub.1 and g.sub.2 change as a result of the laser output. Changes in g.sub.1 and g.sub.2 not only cause .theta.d to change, but may also leave the stability condition in formula (1) unfulfilled, making oscillation difficult.
If resonator length is increased according to (a) above, in order to reduce .theta.d, then a.sub.1 and a.sub.2 increase. Therefore, even a small change in "f" can cause g.sub.1 and g.sub.2 to change significantly, so that targeted performance can be achieved only under a particular condition. Since the technique of using a beam expander according to (b) above amounts to increasing the resonator length, the technique limits the conditions under which targeted performance can be achieved, as in technique (a). Using an unstable resonator according to (c) is very effective for reducing .theta.d, but if the thermal lens effect is large, high performance can be obtained only in limited conditions, as in (a) and (b) above. Technique (d) reduces the thermal lens effect proper, and hence is basically different from techniques (a), (b) and (c). However, this technique has the drawback that .theta.d in the slab-width direction is large. In addition, since a rectangular beam is produced, and as the value of .theta.d varies depending on direction, the technique has difficulty in focusing light into a circular spot. These problems cause inconvenience in applications such as laser cutting, since the cut width varies with the beam's direction of movement.
Technique (e), wherein a very small pinhole is disposed in an output mirror, has been used since the early days of lasers, mainly in gas lasers with small gain. An example is disclosed in the paper by C. K. N. Patel et al., Appl. Phys. Lett., Vol. 4, No. 1 (1964), p. 18. Also, a paper by D. E. McCumber, Bell System Technical Journal, Vol. 44 (1965), p. 333 gives a detailed analysis of a resonance mode in which a hole is provided in an output mirror. A system that uses an output mirror with a hole has a .theta.-value nearly equal to one using a conventional partially permeable mirror, where .theta.d can be reduced by limiting the "d" value. In this case, the transmittance of the output mirror can be determined as the proportion of the pinhole area relative to that of the beam cross-section. However, this relationship holds true only when the pinhole diameter is small. As the pinhole diameter increases, the effective transmittance of the output mirror does not become larger in proportion to the diameter because, with reference to FIG. 3, laser oscillation cannot occur around the center of a laser medium 25 if the diameter of the hole in an output mirror 24 opposite to a total reflection mirror 2 is large relative to the laser medium 25. Therefore, the effective transmittance is smaller than the value determined by the ratio of pinhole area to beam cross-section. For this reason, it becomes more difficult for a laser with large optimal transmittance to achieve the targeted transmittance, and high efficiency cannot be obtained. Also, the presence of a non-oscillating portion reduces efficiency. Furthermore, all solid-state lasers suffer from a common problem relating to admittance of light into an optical fiber. As described above, the diameter of the focused light spot can be made smaller than an optical fiber diameter by reducing .theta.d below a certain targeted value. However, even if this condition is met, a laser beam can only then be admitted into a conventional optical fiber with a diameter of 1 mm or less when the focused light spot matches up with the light-admitting end of the fiber. Such adjustment is difficult and costly.